1. Field of the Invention
This invention relates to a transmission power controller for controlling the transmission power of a mobile station comprising part of a radio communication system. More specifically, it relates to a transmission power controller for use with a code division multiple access (CDMA) communication system.
2. Description of the Background Art
The CDMA communication system has been standardized by the Telecommunications Industry Association TIA) for use in North America and has adapted spread spectrum communication technology. In this system, since a plurality of mobile stations share the same frequency band, signals transmitted by several such stations tend to interfere with each other resulting in degradation of the quality of the signal transmitted by each of the mobile stations. To overcome this problem, a closed-loop power control operation is employed which adjusts the transmission power of each mobile station. Specifically, power control information is transmitted from a base station to each mobile station to minimize interference between the mobile stations.
Referring to the block diagram of FIG. 2, the following is an explanation of the power control operation of a conventional CDMA communication system comprising a base station 100, a mobile station 200, and a base station controller 300.
The base station 100 includes a demodulator 101, a signal-to-noise (S/N) power measuring circuit 102, a power up/down discriminator 103, a power control information multiplexer 104 and a modulator 105. The demodulator 101 demodulates a received signal transmitted from the mobile station 200 via a receiving amplifier 106, and the S/N power measuring circuit 102 measures the received power level of the received signal provided by the demodulator 101.
The power up/down discriminator 103 determines whether the received power level is in a proper or improper status by comparing the received power level with a threshold level provided by a threshold set-up circuit 303 of the base station controller 300 and outputs a power control signal. The threshold level is determined by calculating an error rate of the received signal. For example, when the received power level exceeds the threshold level, the power up/down discriminator 103 outputs a power control bit having a logical value such as xe2x80x9cxe2x88x921xe2x80x9d, which commands a decrease in the transmission power of the mobile station 200. In contrast, when the received power level does not exceed the threshold level, the power up/down discriminator 103 outputs a power control bit having a logical value such as xe2x80x9c+1xe2x80x9d, which commands an increase in the transmission power of the mobile station 200. The power control information multiplexer 104 inserts the power control bit into the transmission signal to be transmitted, the transmission signal is modulated to a spread-spectrum signal for the CDMA communication system by the modulator 105, and the signal is transmitted to an antenna (not shown) via an amplifier 107.
Each mobile station 200 comprises an amplifier 200a, a demodulator 201, a power control information extracting circuit 202, a decoder 203, an up/down decision circuit 204, a transmission amplifier 205, and a transmission circuit 206.
The demodulator 201 demodulates the spread-spectrum signal transmitted from the base station 100 via the receiving amplifier 200a to a baseband signal. The power control information extracting circuit 202 extracts the power control bit from the demodulated received signal. The decoder 203 decodes the demodulated received signal to a voice or data signal. The transmission amplifier 205 is a variable gain amplifier which change its gain as a function of the command signal provided by the up/down decision circuit 204.
The up/down decision circuit 204 is an accumulator which sums the signals received from the power control information extracting circuit 202 during a series of signals received from the base station 100. For example, if five signals, +1, +1, xe2x88x921, +1, +1 are received from the poser control information extracting circuit 202, the up/down decision circuit 204 outputs these command signals, +1, +2, +1, +2, +3, sequentially to the transmission amplifier 205. In response, the transmission amplifier 205 would increase the transmission power by 3dB. In the conventional CDMA communication system, the power control operation is periodically performed every 1.25 micro-seconds. As a result, the mobile station 200 frequently controls the transmission power in accordance with the power control bit.
The base station controller 300 comprises a decoder 301, an error rate measuring circuit 302, a threshold set-up circuit 303 and a coder 304. The decoder 301 decodes the received demodulated signal, provided by the demodulated 101 of the base station 100, to a voice or data signal. The voice or data signal is supplied to the error rate measuring circuit 302 which measures the error rate of the voice or data signal. The threshold set-up circuit 303 determines a threshold level, based on the error rate, which is the transmission power of the mobile station 200 to satisfy the predetermined quality of speech or data signal. The signal threshold level is supplied to the power up/down discriminator 103 of the base station 100. The coder 304 codes a voice or data signal to be transmitted to the mobile station 200.
FIG. 3 is a graph having an abscissa divided into 73 divisions each designating an instant T(n), defined as a sample time, at which the base station 100 transmits a xe2x80x9c1xe2x80x9d or a xe2x80x9c+1xe2x80x9d signal to the mobile station 200 where it is received by the power control information extracting circuit 202 and inputted to the up/down decision circuit 204. The ordinate of FIG. 3 shows variations in the power transmitted by the mobile station 200 to the base station 201, in decibels, at each instant of time T(n). The graph of FIG. 3 illustrates the operation of a typical CDMA communication system in which the power transmitted by the mobile station 200 changes in 1 dB increments at the following sample times: (1) At sample times T(1-10), the transmission power alternates between 1 dB and 0 dB (2) At sample times T(20-53), the transmission power alternates between 10 dB and 11 dB; and (3) At sample times T(60-73), the transmission power alternates between 4dB and 5dB.
During the sample times T(11-21), the base station 100 transmits a series of only xe2x80x9c+1xe2x80x9d signals and therefore the power transmitted by the mobile station 200 gradually increases to 11dB, and during the sample times T(53-60), the base station 100 transmits a series of only xe2x80x9cxe2x88x921xe2x80x9d signals and therefore the power transmitted by the mobile station 200 gradually decreases to 4dB.
It is desirable to keep a fixed transmission power level at the mobile station 200 if the transmission power level matches the threshold level. However, in the conventional CDMA communication system, as shown in FIG. 3, even when the transmission power level matches the threshold level, the mobile station is forced to change transmission power because the base station 100 always sends a transmission power control bit that commands either an increase or a decrease in the transmission power. That is, the power being transmitted by the mobile station 200 is always being changed. As a result, the signal received by the base station 100 does not have a constant received power level.
It is an object of the present invention to provide a transmission power controller that is capable of suppressing deviation in the transmission power at the mobile station. It is an another object of the invention to provide a transmission power controller that is capable of not only suppressing such deviations but also capable of providing a quick response to change in the transmission power when the transmission power level does not match the threshold level.
To accomplish these objectives, a transmission power controller having a transmission amplifier for adjusting transmission power based on power control information signal transmitted from a base station is provided which comprises;
a power control information extracting means for extracting said power control information signal,
an up/down decision means for producing a command signal to control a gain of said transmission amplifier in response to said power control information signal, and
a filtering means for extracting a row frequency band signal from said command signal and for controlling a gain of said transmission amplifier based on said extracted row frequency band signal.